Delft University of Technology
Sm2þ-doped SiAlO sputter deposited coatings for self-absorption and scatter-free
luminescent solar concentrator applications
de Vries, David; van Overbeek, Sadiq; Merkx, Evert; van der Kolk, Erik
DOI
10.1016/j.jlumin.2020.117321
Publication date
2020
Document Version
Final published version
Published in
Journal of Luminescence
Citation (APA)
de Vries, D., van Overbeek, S., Merkx, E. P. J., & van der Kolk, E. (2020). Sm2þ-doped SiAlO sputter
deposited coatings for self-absorption and scatter-free luminescent solar concentrator applications. Journal
of Luminescence, 225(117321), [117321]. https://doi.org/10.1016/j.jlumin.2020.117321
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Journal of Luminescence 225 (2020) 117321
Available online 26 April 2020
0022-2313/© 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Sm
2þ-
doped SiAlO sputter deposited coatings for self-absorption and
scatter-free luminescent solar concentrator applications
David de Vries
a, Sadiq van Overbeek
b,**, Evert P.J. Merkx
a,*, Erik van der Kolk
a,*** aLuminescence Materials, Delft University of Technology, Mekelweg 15, 2629 JB, Delft, the NetherlandsbPhysee R&D BV, Van Mourik Broekmanweg 4, 2628 XE, Delft, the Netherlands
A B S T R A C T
Combinatorial reactive co-sputtering using Al, Si and Sm targets in an Ar þ O2 atmosphere, resulted in Sm doped SiAlO thin films with a wide Sm concentration- and
Si:Al composition gradient. By combining position dependent EDX spectra and laser excited emission spectra, ternary phase diagrams were constructed that directly show the relation between Sm emission intensity, index of refraction, thickness and composition. Using this approach, the Sm2þand Sm3þemission intensity ratio
was controlled towards films with predominantly Sm2þemission, which is most favorable for luminescent solar concentrator (LSC) applications. The optimum Sm2þ
efficiency was reached when the Al content was about equal to the Sm content. When the Si:Al ratio decreases, the Sm2þemission intensity strongly drops to almost
zero. However, sputtering without Al resulted in no Sm2þemission intensity at all. The excitation and emission properties of Sm2þin the optimized thin films,
especially the ratio between the 4f→4f and 5d→4f emission that is sensitively susceptible to the co-ordination polyhedron, closely resembles that of Sm2þdoped
crystalline powders with the same composition. This strongly suggest that the Sm2þions in our amorphous films are coordinated in the same way. A homogeneous
thin film on float glass clearly shows the light concentration effect of the red Sm2þemission. Due to an unexplained low Sm2þabsorption of our films, even the
optimized thin films do not luminesce brightly.
1. Introduction
Luminescent Solar Concentrators (LSCs) have the potential to play a major role in the transition to net zero-energy buildings (NZEBs) when applied as an electricity generating window [1,2]. A promising design for an LSC is a thin film doped with luminescent centers sputter coated on glass. The thin film absorbs part of the solar spectrum and emits this light isotropically. Because of total internal reflection in the film and the glass, this emitted light is concentrated on the edges of the glass. Here, photovoltaic cells convert this concentrated light in electricity. To make the solar concentration process efficient, the thin film should have a strong absorption preferably in the visible spectrum, where 43% of the solar energy is found. Furthermore, the luminescent thin film should have no self-absorption of luminescence [3] and a high quantum yield. As shown by Boer et al. [4], the rare-earth ion samarium (Sm) meets these conditions when doped in the inorganic phosphor SrB4O7. Un-fortunately, it appeared difficult to integrate the phosphor particles in a waveguide that does not scatter light, preventing further progress. In this work we take the next step by incorporating Sm in an industry
compatible, sputtered SiAlO coating on glass.
Various types of scatter-free, silicon-aluminum-oxygen-nitrogen (SiAlON) coatings are widely used in the glass industry as dielectric window coatings, e.g. for anti-reflection and scratch protection. Previ-ously we have shown that these coatings can be further functionalized by doping with Eu2þluminescence centers [5]. While the 5d-states of Eu2þ have a limited absorption spectral range and suffer from self-absorption, the Sm2þ5d-states have a wider absorption range up to 600 nm in the visible spectrum and the 4f→4f Sm2þemission is not self-absorbed by the 5d-states. However, when Sm is doped into the SiAlON sputtered coatings, it occurs not only in the desired divalent (Sm2þ) valence state but also in the trivalent (Sm3þ) state which has its main absorption limited to a narrow UV part of the solar spectrum.
In this research we first report how the thin film composition in-fluences the valence state of the Sm ion using a single deposition of a thin film with a Si:Al composition and Sm concentration gradient. This gradient film was deposited by reactive radiofrequency (RF) magnetron co-sputtering on a non-rotating substrate with three different targets under an angle [6]. Subsequently, position dependent electron * Corresponding author.
** Corresponding author. *** Corresponding author.
E-mail addresses: dtdevries1@gmail.com (D. de Vries), sadiq@physee.eu (S. van Overbeek), e.p.j.merkx@tudelft.nl (E.P.J. Merkx), e.vanderkolk@tudelft.nl
(E. van der Kolk).
Contents lists available at ScienceDirect
Journal of Luminescence
journal homepage: http://www.elsevier.com/locate/jluminhttps://doi.org/10.1016/j.jlumin.2020.117321
Journal of Luminescence 225 (2020) 117321
2 dispersive X-ray spectroscopy (EDX), transmission and laser excited emission spectra were recorded to determine the relation between the composition of the film and the Sm2þand Sm3þluminescent intensities as well as film thickness and refractive index. We will show that the Sm2þ/Sm3þemission intensity ratio can be controlled by tuning the Si: Al ratio. Next we use this control to deposit a second gradient film with lower Al-content with almost exclusively Sm2þemission. Finally, we present a homogeneous film with an optimized Si:Al ratio to qualita-tively demonstrate the Sm2þLSC effect in a film on 100 � 100 � 6 mm3 glass substrate.
2. Methods
2.1. Sample creation
Two SiAlO:Sm thin films were deposited by an AJA ATC Orion 8 magnetron sputtering system with a base pressure of 1� 10 7 Torr (1.3 � 10 10 bar) at room temperature (Fig. 1a). After the targets were installed, the walls of the sputter-chamber were heated to 110 �C for 10 h to remove any remaining vapor. Before deposition, the chamber was DC sputtered with a 2-inch metallic Ti (99.2%–99.7%, Lesker) target for 1 h at 20 W to reduce contamination and water vapor, and as an adhesion layer for other sputtered materials during sputtering, so that they do not flake off the chamber walls.
The thin films were deposited on 50 � 50 � 1 mm3 square UV grade double-sided polished MgF2 substrates (Roditi). MgF2 was chosen as substrate, because glass-type substrates consist of SiO2 which could lead to misinterpretation of EDX measurements. The substrate was sequen-tially rinsed three times with ethanol (96%) and demineralized water. The sputtering was done by three metallic Si (99.999%, Lesker), Al (99.9995%, Lesker) and Sm (99.9%, Demaco) targets, all 5 cm in diameter. These targets were oriented such that the reactively co- sputtered material would reach the substrates from three different sides, as shown in Fig. 1a. On the Sm source a stainless steel mask with a pattern of concentric holes was put covering 85% of the Sm target to reduce the Sm deposition rate. The depositions took place at room temperature and at a working pressure of 3 mTorr (4 � 10 6 bar), sus-tained by gas flows of 18 sccm 5 N purity Ar, introduced at the Al source, and of 1.2 sccm 5 N purity O2, introduced next to the substrate. For the first sample, the guns of Si, Al and Sm operated at powers of 60 W RF, 60 W DC, and 25 W RF respectively for a total duration of 11.25 h. No heat treatment was used during or after deposition of the thin film. The second sample had gun powers of 120 W RF, 20 W DC an 25 W RF on the Si, Al and Sm guns respectively. In this case two masks were used, covering 85% and 60% of the Sm and the Al target, respectively.
2.2. Emission spectra
For the emission spectra, the samples were excited by a wavelength tunable EKSPLA NT230 OPO laser. A grid of 32 � 32 different emission spectra across the sample was measured by using two Thorlabs DDSM100 linear translation stages stacked perpendicularly on top of each other. Longpass filters of 300 nm and 405 nm were used to elimi-nate the reflected laser light of 240 nm and 360 nm used to excite the sample. A 600 μm diameter multimode optical fiber directed the emitted light to a spectrometer (Ocean Optics QE65000), measuring the photo-luminescence with integration times of 1000 ms, averaged 3 times. The emission intensities were corrected for the wavelength dependent effi-ciency and non-linearity of the spectrometer. For further details on the set-up and technique, we refer to Ref. [5].
2.3. Transmission spectra
A Xenon lamp (Hamamatsu, C7535) was used to measure the total transmission through the sample. The light from the Xe lamp is guided through a fiber and collimated (2.7 mm spot diameter) at the sample surface. The light transmitted through the sample is captured by a 5.08 cm diameter integrating sphere (IS200-4, Thorlabs), with an Ocean Optics QEPro spectrometer (200 μm slit width) connected with a multimode optical fiber to the off-axis detector port. The same trans-lation stages used for the emission spectra were used to measure a grid of 24 � 24 transmission spectra across the substrate. The spectra were corrected for the baseline (when no light enters the integrating sphere) and divided by the spectrum measured when the integrating sphere was exposed to direct light from the Xe lamp. The transmission spectra were measured with an integration time of 300 ms and were averaged 20 times.
2.4. EDX
Energy-dispersive X-ray spectroscopy (EDX) measurements were done on 39 coordinates on the first sample and 23 coordinates on the second sample to determine the local concentrations of the chemical elements. A JEOL IT-100 EDX/SEM, operating at a voltage of 15 kV with probe current at 70% and low vacuum mode of 35 Pa pressure provides the possibility of analysis without having to add a conductive layer to the thin film. The sample was attached to the holder with carbon tape and the measurements were performed in Backscattered Electron Shadow (BES) mode. The measurement was conducted at 1000 � magnification.
2.5. XRD
To show the amorphous nature of the thin films, X-ray Diffraction (XRD) measurements were performed. A PANalytical X’pert Pro MPD diffractometer with a Cu Kα anode (λ ¼ 0.1540598 nm) operating at 45 kV and 40 mA for 11 and 1 h was used for the first and second sample respectively. The area illuminated by the X-ray beam was around 1 � 5 mm2 in size and was measured in the middle of the samples. All of our thin films appeared to be amorphous. An example XRD spectrum is shown in the supplementary information section.
2.6. Quantum efficiency
The external quantum efficiency defined as the ratio of emitted photons to the absorbed photons by the LSC sample was measured ac-cording to the method described in Refs. [7]. As an excitation source, an EKSPLA NT230 OPO laser set to 360 nm was coupled in a 5.3 inch Labsphere integrating sphere. The resulting spectra were measured at on off-axis detector port with an Ocean Optics QEPro spectrometer with an integration time of 400 ms and averaged over 80 scans.
Fig. 1. Fabrication of the compositional library of a SiAlO:Sm sample with gradients of Si, Al and Sm. (a) Photograph of the sputter set-up showing the four guns. A mask is placed on top of the Sm gun. The Ti gun is not used for fabrication of the samples. (b) Photograph of the first sample. The thin film is transparent and under this angle the interference fringes can be seen. The Si, Sm and Al-rich sides are on the left, top and right respectively.
2.7. Experimental results
To realize a SiAlO:Sm thin film, the substrate was sputtered from three different sides by Si, Sm and Al sputter guns. By not rotating the sample during deposition, a concentration gradient of the three elements was established. Fig. 1a shows the sputter gun configuration. The sub-strate is located above the guns. The Ti target is not used for fabrication of the sample but rather to improve the vacuum of the sputter chamber. Fig. 1b shows the as-deposited thin film where Si is sputtered from the left side (x ¼ 0 mm), Sm is sputtered from the top side (y ¼ 0 mm), and Al is sputtered from the right side (x ¼ 50 mm). In all following figures, the orientation of Fig. 1b will be used. The sputtered SiAlO:Sm thin film is transparent and shows interference fringes that arise due to the difference in refractive index of the film and the substrate and due to the thickness gradient. Because the Al gunsettings resulted in a higher sputtering yield than the Si gun, the interference fringes follow the Al sputtering direction. The roundness of the fringes originate from the radial sputter distribution of the targets.
Using an xy-scanning set-up [6], transmission measurements were performed on 24 � 24 distinct coordinates on the sample. Each trans-mission spectrum was fitted with a model as explained in Ref. [5,6]. This model yields the thickness and refractive index of the film as a function of position. Fig. 2a shows the film thickness in μm, which ranges from 2.8 μm on the Si-rich side to 4.8 μm on the Al-rich side. The thickness is zero in the bottom-left and top-right because no coating is applied here due to the clips that fixed the substrate in place.
Fig. 2b shows the refractive index of the film. The refractive index ranges between 1.53 on the Si-rich side of the film to 1.60 on the Al-rich side. This is expected, as the Al-rich side would be closer to a Al2O3 composition, which has a higher refractive index (n ¼ 1.77) [8] than SiO2 (n ¼ 1.46) [8]. Sm2O3 has a refractive index of n ¼ 1.93 [9], which can explain the observation that the refractive index increases towards the top-right corner of the film.
To determine the local concentrations of the Si, Al and Sm cations, EDX measurements were performed on 39 coordinates of the thin film. The white dots in Fig. 3a indicate the 39 locations where EDX mea-surements were performed. The surface-source equation [5,10] was used to interpolate between the 39 coordinates to get a continous dis-tributions. Fig. 3a, b and c show the Si, Sm and Al cation concentration respectively. Since Si was sputtered from the left, Sm from the top and Al from the right, as expected, the highest Si concentration is found on the left, the highest Sm concentration in the top-left, and the highest Al concentration is found on the right. To obtain a higher Al concentration compared to Si, Al was DC sputtered and Si was RF sputtered. The dif-ference in sputtering yields of Al and Si also results in the shape of the Sm gradient. Fig. 3d, e and f show the Si, Sm and Al cation concentration respectively of a sample that will be discussed later. The areas in Fig. 3g show the range of cation concentrations in % in a ternary diagram. The blue area in the bottom right corner describes the concentration range of
this sample, the red area in the bottom left describes a range of the sample which will be discussed later. In this sample, Si ranges from 4.0 to 37.5 at.%, Sm ranges from 1.6 to 9.5 at.%, and Al ranges from 56.0 to 94.4 at.%. Note that by using a larger substrate or by depositing multiple films the entire ternary diagram can be filled.
Emission spectra were recorded at 32x32 distinct coordinates across the substrate at an excitation wavelength of 360 nm, mostly exciting Sm2þions in their 5d-state. Gaussian distributions were fitted to all 32 � 32 emission spectra, of which one example is shown in Fig. 4a. Fig. 4b and c show emission spectra at some of these locations. The sharp emission peaks as shown in Fig. 4a at wavelengths of 682 nm, 700 nm, 725 nm, 761 nm, and 809 nm are assigned to the 5D0 → 7F0, 5D0 → 7F1, 5D0 → 7F2, 5D0 → 7F3, and 5D0 → 7F4 of Sm2þ[11] respectively. The 5D0 → 7F0 transition is the only nondegenerate 4f6 → 4f6 transition. Hence, it is the sharpest. Next to the peaks associated with Sm2þ, peaks with lower intensity related to 4f5 → 4f5 transitions in Sm3þat 599 nm and 646 nm are observed. These peaks are assigned to the 4G5/2 → 6H7/2 and 4G5/2 → 6H
9/2 transitions respectively. Furthermore, two broad bands between 600 and 900 nm can be observed which we tentitively assign to Sm2þ 4f55d → 4f6 emission.
By correlating the concentrations of Fig. 3 to the position dependent 5D0 → 7F0 emission intensity of the fitted peaks, the emission intensities could be directly related to the local compositions. Fig. 4d shows the result of this correlation in a zoomed-in ternary diagram. The emission is most intense in the area where the Si concentration is highest, the Al concentration is lowest, and the Sm concentration is lowest.
Fig. 4d includes 3 so-called Sm iso-lines with a constant Sm con-centration but changing Al:Si ratio and 3 Al:Si iso-lines where the Sm concentration varies. The 4f6 → 4f6 line emission intensity appears to decrease with increasing Sm concentrations for all Al:Si ratios. We assign this drop in intensity with increasing Sm concentration to a concentration quenching mechanism that that has been observed before in Sm2þdoped phospors [12–14].
The ternary phase diagram also provides the opportunity to study the Sm2þemission intensity as a funcion of the Al:Si ratio while keeping the Sm at.% constant. In this case, the intensity needs to be monitored along horizontal lines in the phase-diagram. As can be seen in the inset of Fig. 4b, it follows for example that for a Sm concentration of 5 at.% the emission intensity decreases by a factor of 60 when the Si/(Si þ Al) ratio changes from 0.38 to 0.07, showing the strong dependence of emission intensity on local composition. The strongest Sm2þemission is found in a Si-rich environment where the Sm concentration is the lowest.
The same approach can be repeated at an excitation wavelength of 240 nm at which mostly the Sm3þcharge transfer band is excited. Fig. 5a shows an example of the Gaussian functions that were fitted to the emission peaks at all 32 � 32 locations. In Fig. 5b and c, emission spectra are shown for different locations on the substrate. In contrast to 360 nm excitation, we mainly observe Sm3þ: 4f5 → 4f5 emission. The peaks at wavelengths of 562 nm, 599 nm, 646 nm and 710 nm are assigned to 4G
5/2 → 6H5/2, 4G5/2 → 6H7/2, 4G5/2 → 6H9/2 and 4G5/2 → 6H11/2 tran-sitions of Sm3þrespectively [11]. Fig. 5d shows a ternary diagram of the integrated intensities of the 4G5/2 → 6H9/2 emission peaks directly relating composition to Sm3þemission intensity.
The ternary diagram of Fig. 5d can be used to study the Sm3þ emission intensity as a function of the Sm concentration for constant Al: Si ratio as well as as a function of the Al:Si ratios for constant Sm con-centrations. The PL intensity increases linearly by a factor of about 2 for constant Al:Si ratio when the Sm concentration decreases from 5.5 to 2.6 at.% (at Al:Si ¼ 90:10, see the inset of Fig. 5c). This increase in emission intensity with increasing concentration is again ascribed to concentration quenching. The Sm3þ emission intensity increases by about a factor of 2 when the Si/(Si þ Al) ratio decreases from 0.38 to 0.07 at 5 at.% Sm concentration (see the inset of Fig. 5b). This intensity decrease with Si/(Si þ Al) ratio is much smaller compared to the 60 times decrease for Sm2þ emission mentioned earlier. The emission properties of Sm3þseem to be less sensitive to the local composition Fig. 2. Properties of the thin film library. (a) The position-dependent thickness
of the deposited thin film in μm. (b) The position-dependent refractive index of the thin film.
Journal of Luminescence 225 (2020) 117321
4 compared to Sm2þemission and is strongest in the Sm-poor and Al-rich portion of the composition range.
The ternary phase diagrams presented in Figs. 4d and 5d predict that a thin film with optimized Sm2þemission intensity and minimal to no Sm3þemission can be made by drastically increasing the Si:Al ratio. For this reason a gradient SiAlO:Sm thin film was sputtered with concen-trations in the Si-rich corner of the ternary diagram as shown by the red area in the bottom left corner of the ternary diagram in Fig. 3g. A grid of 20 � 20 emission spectra were measured under 360 nm excitation. Fig. 6a shows the excitation and emission spectra measured after the sample had been annealed. Almost exclusively Sm2þ emission is observed and the shapes of the emission spectra are identical to those measured in the first sample. To link the emission intensities to the local concentrations, 23 EDX measurements were performed and interpolated over the substrate by the same method as applied to the first sample (see
Fig. 3d). The resulting ternary diagram can be found in Fig. 6b. Si ranges from 68.0 to 92.0 at.%, Sm ranges from 4.7 to 15.4 at.% and Al ranges from 3.3 to 16.6 at.%. The optimized emission intensity is found for a Si: Al ratio of 23:1, or more specifically, for a concentration ratio of 91.5 at. % Si, 4.7 at.% Sm, and 3.9 at.% Al. This result raised the question whether Al is needed at all for an optimal Sm2þemission. Thus, a sample was sputtered which had identical sputtering settings apart from the fact that no Al was co-sputtered. After exciting this sample at 360 nm, no Sm2þnor Sm3þemission could be observed. It is therefore concluded that small amounts of Al are needed for the sample to exhibit Sm2þ emission.
To demonstrate the LSC application, a SiAlO:Sm2þ thin film was sputtered onto a 100 � 100 � 6 mm3 float glass substrate by utilizing the outcomes of the second sample discussed above. After annealing the substrate, it was put under a UV light source. A photograph shown in Fig. 3. Library of the position-dependent local con-centrations for both samples discussed in this article. (a)&(d) The Si concentrations (at.%). The white dots indicate the locations on which EDX measurements were performed. (b)&(e) The Sm concentrations (at. %). (c)&(f) The Al concentrations (at.%). (g) The achieved concentrations of the first sample (bottom right blue area) and the second sample (bottom left red area). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 4. Emission spectra of the first sample excited at 360 nm. (a) The deconvolution of a Sm2þemission spectrum. (b) Five emission spectra measured along a Sm- isoline of 5 at.% Sm. An overview of the emission intensities as a function of Si/(Si þ Al) ratios for three different Sm at.% are depicted in the inset. (c) Another three emission spectra measured along an Al:Si-isoline of Al:Si ¼ 80:20. In this case the emission intensities as a function of Sm at.% are displayed in the inset for 3 different Al:Si ratios. (d) A ternary diagram directly relates the Sm2þemission intensities to the local concentrations. The numbers on the contour lines indicate relative emission intensities.
Fig. 5. Emission spectra of the first sample excited at 240 nm. (a) The deconvolution of a Sm3þemission spectrum. (b) Five emission spectra measured along a Sm- isoline of 5 at.% Sm. An overview of the emission intensities as a function of Si/(Si þ Al) ratios for three different Sm at.% are depicted in the inset. (c) Another three emission spectra measured along an Al:Si-isoline of Al:Si ¼ 70:30. In this case the emission intensities as a function of Sm at.% are displayed in the inset for 3 different Al:Si ratios. (d) A ternary diagram directly relates the Sm2þemission intensities to the local concentrations. The numbers on the contour lines indicate relative emission intensities (scaled to this figure, these numbers should not be used to compare intensities with those reported in Fig. 4).
Journal of Luminescence 225 (2020) 117321
6
Fig. 7 shows that the UV light is re-emitted by Sm2þin the red part of the spectrum, transported within the glass and it exits the glass at the edges, where the concentrated red light is clearly visible. The optical efficiency of the LSC is however still limited due to weak absorption by Sm2þand an external quantum efficiency of ca. 4.5%.
3. Discussion
The Sm2þ4f6→4f6 emission after 4f6→4f55d excitation at 360 nm is due to an interconfigurational energy transfer from the lowest energy relaxed Sm2þ5d states to the excited Sm2þ4f levels [15].
We observe that when the 4f6→4f6 emission intensity from Sm2þ decreases, also the broad emitting bands between 600 and 900 nm also decrease. These broad bands do not show a similar correlation with the f→f transitions coming from Sm3þ. We therefore ascribe these broad bands to 4f55d→4f6 emission from Sm2þ. Furthermore, we observe that these band not only retain the same intensity ratio to the 4f6→4f6 emission lines from Sm2þ, this 4f55d→4f6 emission also occurs at the same wavelength, i.e. energy, irrespective of the Si:Al ratio in our sample. Both the efficiency of the 4f55d→4f6 transition, expressed as intensity, as well as the energy at which this transition occurs are sensitively susceptible to the coordinating polyhedron of Sm2þ. A small change in coordiating polyhedron would be expressed in a shift in en-ergy of the 4f55d→4f6 transition, or a shift in its intensity relative to the 4f6→4f6 lines. These observations can be therefore be explained by assuming that Sm2þions can only emit efficiently in one specific coor-dinating polyhedron that has a strong tendency to form, irrespective of the composition.
This suggestion is supported by the fact that Sm emission in phos-phors, with varying composition and phases like mullite or crystoballite, again show the same intensity ratio despite the fact that films and powdered phosphors are made in an entirely different way. A compar-ison of the emission of our films and our powdered phosphors can be found in the Supplementary Information.
As our films are amorphous it is difficult to determine how the Sm ions enter the SiAlO host and deduce the local coordination around Sm in terms of size and symmetry of the Sm site. Our main observations can however be qualitatively understood by a comparison with extensive published work on SiO2 glasses doped with rare earth ions [16] including Sm [14,17–20] in relation with the development of lasers, optical memories or amplifiers.
Our results show that the intensity of the emission changes dramatically as the local composition of our film changes. The intensity of Sm emission in SiO2 glasses is strongly controlled by the poor solu-bility of Sm and rare earth ions in general, because there is no regular site for Sm in the compact SiO2 amorphous structure with closely condensed SiO4 structural units.
Nogami et al. [14] for example showed that concentration quenching already takes place at a much lower concentration than was expected based on a homogeneous distribution of doping ions, suggesting the tendency of Sm ions to form Sm2O3 or Sm metallic clusters. The same conclusions were drawn for other rare earth ions [21]. Talbot et al. [22] explicitly visualised these clusters by atom probe techniques in Er doped fibres. Our transmission spectra, presented in the supplement section, confirm the poor Sm solubility. The absorption spectra indeed show that we have very low absorption intensity (<1%) despite relatively thick (3–4 μm) and highly doped (2–8 at.%) films. This is very much in contrast to our recent work on Eu2þdoped SiAlON thin films, sputtered under similar conditions, where we have reported about 30–50% 4f6→4f55d absorption in 400–600 nm thick gradient thin films with about 10 at.% of Eu2þdoping [5]. Thus we conclude that our films have much lower Sm2þ doping % than the Sm content shown in Fig. 2, established by EDX. Our data does not allow us to conclude whether Sm is included as Sm2O3 or metallic clusters instead as suggested in other work mentioned above. This is in line with typical published concen-tration in SiO2 glasses that are never higher than 1 at.% [14,17,19]. Fig. 6. Emission spectra of a sample with a higher Si content. (a) Emission
spectra recorded at the locations indicated by the circles in the lower phase diagram in b. (b) Ternary diagrams directly relating the Sm2þemission in-tensity to the local Si:Al:Sm concentrations. The numbers on the contour lines indicate relative emission intensities (scaled to this figure, these numbers should not be used to compare intensities with those reported in Fig. 4
or Fig. 5).
Fig. 7. Photograph of the demonstration LSC sample under UV illumination.
Solubility of Sm can however be enhanced by co-doping with Al. In addition co-doping with Al promotes the Ln3þ→Ln2þtransition in case of Yb and Sm [14,23].
Morimo et al. [17], inspired by earlier work on Nd3þby Arai et al. [24] was the first to observe a 10-fold enhancement of Sm3þfluorescent Intensity in a SiO2:1%Sm glass by 10% AI co-doping. Their EXAFS study, combined with an assumed concentration quenching of the Sm3þ emission, revealed that at higher Sm concentration, Sm agglomeration in the form of an amorphous Sm oxide phase takes place. In addition it was shown that Al co-doping ions are preferentially coordinating the Sm ions in a local shell. It was therefore concluded that the enhanced fluorescence by Al co-doping was due to a reduction of Sm agglomera-tion and the corresponding reducagglomera-tion in concentraagglomera-tion quenching [17]. Furthermore Jin et al. [19] concluded, based on NMR studies, that the incorporation of Sm into 10Al2O3–90SiO2 glass promotes the formation of AlO4 structural units at the sacrifice of AlO6 structural units. By comparing NMR data of mullite crystals it was suggested that when Sm2þor Sm3þenters the silica structure the extra positive charge is compensated by extra negative charge by replacing SiO4 units with AlO4 units. It is suggested that Sm ions enter at similar large oxygen vacancy sites identical to mullite [14,19] because of the large ionic radius of Sm3þ and Sm2þ and related large coordination number. The same conclusion was drawn for other RE ions in SiAlO glasses like Eu2þ[25], Ce3þ[26] and Er3þ[27].
The composition of our Sm2þ films with optimized intensity was 91.5 at.% Si, 4.7 at.% Sm, and 3.9 at.% Al. This is very close to the composition of well-studied amorphous SiO2 glass fibers, co-doped with Sm and Al described above. Hence, it is justified to apply knowledge about the glasses to our films. It is clear from our study that Sm2þhas extremely low solubility in SiO2 thin flms as no luminescence could be observed when no Al ions were co-sputtered. Only with Al co-sputtering, Sm2þemission could be observed. As can be seen in Fig. 6b, the Sm2þ intensity reaches an optimum at a 3.9 at.% Al. due to a enhencement of the solubility of Sm. A further increase of the Al at.% lowers the Sm2þ intensity significantly. Apparently, when the Al concentration increases to much higher values than the Sm2þconcentration, the amorphous host no longer supports the favorable sites for the large Sm2þions causing the Sm2þemission to drop.
Although our film shows luminescence as deposited, the intensity can be enhanced by annealing at 650 �C. Futugami et al. [18] was the first to report on Sm doped thin films made by RF magnetron sputtering of a SiO2–Al2O3–Sm2O3 target sintered at 1490 ᵒC at 16.9 MPa. Their films showed weak Sm3þand no Sm2þluminescence as deposited. Sm2þ emission only appeared after heating at 300 ᵒC in air, while the Sm3þ emission remained constant. It was explained by an “optical activation” during which the thermal energy enables the above described local co-ordination of Sm. Zanatta et al. [15] made SiO2 thin films by RF sput-tering of a Si target with Sm2O3 on top. Their sub-oxide SiOx films showed the strongest emission after annealing in air at 1000 ᵒC. This was explained by a widening of the SiO2 bandgap and a drop in the number of non-radiative centers in the bandgap. As our films are almost stoi-chiometric (SiO2), it is likely that film annealing in our case causes a similar optical activation as described in Ref. [18] through a structural rearrangement around the Sm2þions, resulting in the earlier described Sm2þsite in the amorphous structure.
Finally, a note on the observed Sm3þemission. Fig. 4 shows that the optimum Sm3þ intensity under 240 nm excitation is at the highest observed Al concentration in our films of more than 90 at.%. At these high Al concentrations the structure becomes more like Al2O3 as opposed to SiO2 in which the Sm ion prefers to be in the trivalent state. Similar explanations are given for e.g. Eu in SiO2 [25,28]. Given the intensity map of the Sm3þemission presented in Fig. 4 we may conclude that the Sm3þemission in our films is from Al2O3 related sites in our amorphous films.
4. Conclusions
Scatter-free coating on glass with Sm2þemission can be made using an industry compatible reactive sputtering method. The Sm2þ5d-states have a wide excitation range up to 600 nm in the visible spectrum and the 4f6→4f6 Sm2þemission is not self-absorbed by the 5d-states due to inter-configurational non-radiative 4f55d→4f6 relaxation. This makes Sm2þin SiAlO promising candidate materials to realize an electricity generating window based on the LSC principle. From the composition independent constant ratio of 4f6→4f6 and 4f55d→4f6 emission in-tensity, we propose that the Sm2þemission is always from the same Sm2þ site in the SiAlO amorphous structure. Sm valence can be controlled in the desired 2þ state and its intensity optimized by tuning the Si:Al ratio to 23:1. Without any Al co-doping no Sm2þemission is observed. An unexplained low Sm2þ4f55d→4f6 absorption is found, despite a Sm doping concentration up to 9% and 4.5 μm thick layers. Based on earlier work, we speculate that metallic Sm or Sm2O3 nano- inclusion might cause this low Sm2þ4f55d→4f6 absorption. Because of the resemblance between our thin-film data and data on Sm2þdoped crystalline phosphors, we suggest that Sm ions enter at the large oxygen vacancy sites identical to that in mullite and SiO2 glasses.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
CRediT authorship contribution statement
David de Vries: Conceptualization, Software, Formal analysis,
Investigation, Data curation, Writing - original draft, Visualization.
Sadiq van Overbeek: Conceptualization, Validation, Writing - original
draft, Writing - review & editing, Supervision. Evert P.J. Merkx: Methodology, Software, Validation, Formal analysis, Data curation, Writing - review & editing, Visualization. Erik van der Kolk: Concep-tualization, Writing - original draft, Writing - review & editing, Super-vision, Project administration, Funding acquisition.
Acknowledgements
The authors would like to acknowledge Sander Blom for assisting with the quantum efficiency measurements and Joe Kao for providing us the emission spectra of SiAlON:Sm phosphors. EM acknowledges sup-port from the Netherlands Organization for Scientific Research (NWO/ OCW), through the LumiCon project (15024).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi. org/10.1016/j.jlumin.2020.117321.
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